1. Field of the Invention
This invention relates to a lithographic projection apparatus and systems and methods for measuring stray light in a lithographic projection apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs), and other devices involving fine structures.
In some lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the substrate. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a semiconductor wafer). The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the mask's pattern, via imaging, onto a layer of radiation-sensitive material (photo-resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Instead of a mask, in some lithographic apparatus, the patterning device can be a patterning array that comprises one or more arrays of individually controllable elements. Sometimes, the pattern can be changed more efficiently in a maskless system compared to a mask-based system. These types of apparatus are referred to as Optical Maskless Lithographic (OML) apparatus.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a subsequent exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a subsequent exposure.
In order to achieve optimum performance in a mask-based or OML apparatus, monitoring the effect of stray light is important.
Stray light is measured on lithography systems using two primary methods: the Stray light At Multiple Object Size (SAMOS) test and the Kirk test.
The SAMOS test is a radiometric measurement that positions an image-plane detector (i.e., a detector placed in the wafer plane) within the image of a dark box surrounded by a bright field. The detector aperture is usually smaller than the image of the dark box. The light falling on the detector is an indication of amount of stray light present in the system. Details about the SAMOS test are described in U.S. Pat. No. 6,862,076 B2, which is incorporated herein by reference in its entirety.
The Kirk test is a lithographic measurement involving printing dark boxes (surrounded by bright fields, similar to the SAMOS test) into a photo-resist at increasing radiation or dose levels. The dose at which a dark box is not imaged into the photo-resist gives a measure of the stray light present in the system. In some embodiments, the Kirk test is implemented in positive photo-resist with multiple dark box sizes. Intensity is increased until the boxes of interest disappear. So, this test is also referred to as the “disappearing box test.” The Kirk test is not limited to positive photo-resist. For example, when a negative photo-resist is used, intensity is increased until boxes of interest appear.
Both the SAMOS test and the Kirk test are valuable tools for measuring the performance of lithography apparatus in the presence of stray light, but SAMOS is often used for its relative simplicity, because no wafer processing is involved. However, either of the SAMOS test and the Kirk test alone does not provide an accurate measure of stray light in a lithographic apparatus aiming to achieve a very high contrast and a desired critical dimension of features. Additionally, SAMOS test is not a reliable method to measure stray light, where an imaging field of the lithography apparatus is small for each step of exposure, for example, in an OML apparatus.